Laser imaging systems are commonly used to produce photographic images from digital image data generated by magnetic resonance (MR), computed tomography (CT) or other types of scanners. Systems of this type typically include a continuous tone laser imager for exposing the image on photosensitive film, a film processor for developing the film, and an image management subsystem for coordinating the operation of the laser imager and the film processor.
The digital image data is a sequence of digital image values representative of the scanned image. Image processing electronics within the image management subsystem processes the image data values to generate a sequence of digital laser drive values (i.e., exposure values), which are input to a laser scanner. The laser scanner is responsive to the digital laser drive values for scanning across the photosensitive film in a raster pattern for exposing the image on the film.
The continuous-tone images used in the medical imaging field have very stringent image-quality requirements. A laser imager printing onto transparency film exposes an image in a raster format, the line spacing of which must be controlled to better than one micrometer. In addition, the image must be uniformly exposed such that the observer cannot notice any artifacts. In the case of medical imaging, the observers are professional image analysts (e.g., radiologists).
Film exposure systems are used to provide exposure of the image on photosensitive film. Known film exposure systems include a linear translation system and a laser optical scanning assembly. The laser scanning assembly includes a laser scanner with unique optical configurations (i.e., lenses and mirrors), for exposure of the image onto the film. The linear translation system is a mechanical device that converts rotary motion into linear motion. The linear translation system provides for movement of the laser scanning assembly in a direction perpendicular to the scanning direction, such that a full image may be scanned onto a piece of photosensitive film.
In an internal drum type laser scanner assembly, a piece of film is positioned onto a film platen, wherein the film platen has a partial cylindrical or partial drum shape. The photosensitive film is positioned against the film platen. The laser optical scanning assembly is positioned at the center of curvature of the photosensitive film for scanning a scan line across the photosensitive film surface. The linear translation system moves the laser optical scanning assembly lengthwise along a longitudinal axis as defined by the center of curvature of the film to expose an entire two-dimensional image onto the film, consisting of a series of small dots known as pixels.
Typically, the laser optical scanning assembly includes a rotatable scan optic that consists of a single mirror, an assembly of more than one mirror, or a glass prism. The rotatable scan optic is commonly mounted on a shaft that is supported by some type of bearing assembly which includes radial and thrust bearings. The shaft itself is ultimately rotated by a motor which is driven by a controlling electronics system.
A drawback of this type of scanning assembly is that any inaccuracies in the fabrication of the bearings, shaft and/or motor result in "wobble" along the rotational axis of the rotatable scan optic. This mechanically induced wobble translates into a beam wobble causing laser beam pointing errors in a cross scan direction. These laser beam pointing errors ultimately result in image defects in the exposed image that adversely affect the quality of the resulting developed film image. Therefore, to achieve high quality, exacting images requires that the bearings, shaft and motor be fabricated from high quality materials using extremely precise manufacturing techniques. This translates into undesired high production costs for fabricating scanner assemblies for laser imaging systems.
Even though high quality materials and extremely precise manufacturing techniques can produce scanner assemblies that minimize wobble and generate high quality developed images, the components of these scanner assemblies still exhibit normal wear over time. For example, in order to obtain a high resolution image on the photosensitive film in as short a time as possible, it becomes necessary to rotate the rotatable scan optic at a very high rotational rate of speed. This speed is typically on the order of twenty thousand revolutions per minute (i.e., 20,000 rpms). This high rotational rate of speed can cause wear to the bearings, shaft and/or motor of a scanner assembly that can produce wobble causing image defects in the exposed image that adversely affect the quality of the resulting developed film image.
There is a need for an improved scanner assembly for use in a laser imaging system. In particular, there is a need for a scanner assembly that corrects for wobble along the rotational axis of the scan optic, so as to substantially eliminate image defects in the exposed image that would otherwise adversely affect the image quality of the developed photosensitive film. In addition, the scanner assembly should provide these features while being relatively easy to manufacture using lower cost scanner assembly components.